Field of the Invention
The present invention relates to electrolytes that have very wide electrochemical stability window, and can therefore support Li ion chemistries that occurs near or above 5.0 V. More particularly, this invention relates to the compounds that can be incorporated into electrolytes as electrolyte co-solvents, electrolyte additives, or electrolyte solutes, the result of such incorporation being that the electrolytes can support the reversible Li ion intercalation/de-intercalation chemistry at potentials above 4.5 V. Still more particularly, this invention relates to the compounds that can be incorporated into the electrolyte as electrolyte co-solvents, electrolyte additives, or electrolyte solutes, which, upon the initial charging of cathode, decompose sacrificially to form a passivation film. This passivation film prevents sustaining decomposition of electrolyte components but does not hinder the reversible Li ion intercalation/de-intercalation chemistry at potentials above 4.5 V.
The invention of such an electrolyte will enable the use of high voltage cathode materials, affording new rechargeable battery chemistries with higher energy density as well as delivering energy of higher quality in the form of direct electricity current at higher voltages, which are unavailable otherwise from the state-of-the-art electrolytes. The state-of-the-art electrolytes, comprising mainly organic carbonate esters, decompose at potentials below 4.5 V on those high voltage cathode surfaces and cause sustaining capacity fading accompanied with increasing cell impedances.
The said high voltage cathodes include, but are not limited to, transition metal-oxides with spinel lattice structures or metal phosphates with olivine lattice structures, or metal fluorides with conversion reaction natures.
More particularly, the novel compounds of the present invention goes beyond the battery application and could benefit any electrochemical devices that pursue higher operating potentials. The presence of the novel compounds in the electrolyte can stabilize the highly oxidizing surface of the positive electrode and hence enable new chemistry that is otherwise impossible with the current state-of-the-art electrolyte technology. Such electrochemical devices include, but are not limited to, rechargeable batteries, double layer capacitors, pseudo-capacitors, electrolytic cells, fuel cells, et cetera.
Still more particularly, the batteries or the electrochemical devices comprise, but are not limited to, (1) an anode such as lithium or other alkaline metals, alloys of lithium or other alkaline metals, intercalation hosts such as layered structured materials of graphitic, carbonaceous, oxides or other chemical natures, non-intercalating hosts of high surface area or high pseudo-capacitance, et ceteras; (2) a cathode such as an intercalation host based on metal oxides, phosphates, fluorides or other chemical natures, or non-intercalating hosts of high surface area or high pseudo-capacitance, et ceteras; and (3) an electrolyte of the present invention. These electrolytes comprise (a) one or more electrolyte solutes with various cations and anions, (b) a solvent or a mixture of solvents based on organic carbonates or other compounds, and (c) one or more additives. Any of (a), (b) and (c) could be from the claimed structures of the present invention.
Description of the Prior Arts
Li ion chemistry is established upon reversible intercalation/de-intercalation of Li ion into/from host compounds. The voltage of such an electrochemical device is determined by the chemical natures of anode and cathode, where Li ion is accommodated or released at low potentials in the former, and at high potentials in the latter. Apparently, the reversibility of the cell chemistry and the resultant energy density are limited by the stability of the electrolyte to withstand the reductive and oxidative potentials of these electrodes. In today's market, a majority of Li ion batteries use organic carbonate as electrolyte solvents, which decompose oxidatively above 4.5 V vs. Li, and set an upper limit to the candidate cathode chemistry. Despite the fact that 5 V Li ion chemistry has already been made available from such cathodes like olivine structured LiCoPO4 (˜5.1 V) and spinel structured LiNi0.5Mn1.5O4 (˜4.7 V), their advantages such as high energy density and quality cannot be realized due to the lack of an electrolyte that is able to withstand high voltage operation.
Early attempts have been made by one of the inventors to identify an electrolyte system that can resist oxidation beyond 5.0 V, and unsymmetrical sulfones were shown to be such a system on spinel LiMn2O4 surface (K. Xu, et al, J. Electrochem. Soc., 1998, Vol. 145, L70; J. Electrochem. Soc., 2002, Vol. 149, A920). However, intrinsic shortcomings of sulfone as a major electrolyte component, including its failure to form a protective layer on graphitic anode, slow Li ion kinetics, and poor electrode active material utilization caused by high viscosity, prevented wide application.
Additional improvements were also made on mitigating the oxidizing nature on the cathode surfaces through surface coating approaches, and various metal oxides or phosphates were shown to be effective in elongating the service life of the carbonate-based electrolytes (J. Liu, et al, Chem. Mater, 2009, Vol. 21, 1695). But these coating approaches have their own intrinsic shortcomings as well. They not only add additional cost to the manufacturing of the cathode materials, but also induce further interphasial resistance to the Li ion migration at electrolyte/cathode junction. Moreover, overall coverage of cathode particle surface with those inert coatings will inevitably decrease the energy density of the device.
It is therefore of significant interest to the battery industry to find a technology that can effectively enable the 5.0 V class cathode to be applied in Li ion batteries, without the aforementioned shortcomings.
To be more specific, it is therefore of significant interest to the battery industry to find a technology that can effectively enable the 5.0 V class cathode to be applied in Li ion batteries, while there is no major negative impact on the original electrolyte and cathode materials. Such negative impact have been exhibited in the prior arts, and include but are not limited to, the failure of electrolyte to form desired interphasial chemistry on graphitic anode, the slowed Li ion kinetics and difficult electrode wetting due to high electrolyte viscosity, the increased electrolyte/cathode interphasial impedance, additional processing cost of material manufacturing, and sacrificed cathode energy density, et cetera.
It is therefore still of significant interest to the battery industry to identify such electrolytes that can stably support reversible Li ion chemistry, without those shortcomings exhibited by the prior arts.
It is therefore still of significant interest to the battery industry to identify such compounds that, once incorporated as an electrolyte component, can assist in forming a protective layer on the surface of the 5.0 V class cathodes.
It is therefore still of significant interest to the battery industry to identify such compounds that could serve the aforementioned purposes either as electrolyte solvent, co-solvent, solute, or both molecular and ionic additives.
This invention will provide such a technology of the electrolytes with all those desired merits.